As a size of a MOS transistor is reduced, maintaining appropriate capacitance between the gate and the channel while avoiding direct tunneling current through the gate insulation film is a problem. Especially, where a silicon oxide film is used as the gate insulation film, the silicon oxide film should be thin enough for appropriate capacitance but the thin silicon oxide film may result in direct tunneling current.
Therefore, a high-k film having a high dielectric constant (k) is sometimes used as a gate insulation film because it allows maintaining the capacitance between the gate and the channel with a relatively thick layer.